Freaky Ways Animals Woo Mates With Gifts

Across the land on Valentine's Day, lovers and partners exchange gifts. They're not alone. Many animals give gifts, too, and in some ways they're not so very different from us.

After all, from a certain perspective, a box of chocolates is just another term for "materials beyond the obligatory gametes that are transferred from one sex to another during courtship or mating."

That definition comes from "The Evolution of Animal Nuptial Gifts," a review of animal gift-giving written by Tufts University biologists Sara Lewis and Adam South, who are among the few researchers to study the behaviors.

Compared to courtship-related features like flashy ornamentation and wild dances, gift-giving gets short scientific shrift, though not for lack of importance. "There are just so many different kinds of things that males give to females while they're mating," said Lewis. "Nuptial gifts have evolved in so many different lineages. The big mystery is, what's going on here?"

From firefly spermatophores to bowerbird dens to the love darts of snails, Wired takes a Valentine's Day tour of gift-giving in the animal kingdom.

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Love Darts

As with many animal species, it benefits a snail — not a male or female, as they're hermaphroditic and play both roles during mating, but any snail — to be picky about the sperm with which they're fertilized. Snails are thus able to reject the sperm of certain mates, while letting others through. Conversely, it benefits the sperm-providing male to find a way of encouraging the sperm's passage against a partner's disinclination.

Evolution's solution: Snail love darts, the formal term for calcium-shelled harpoons that land snails fire into each others' skin during mating. Inside the darts are chemical cocktails that manipulate a partner's hormones, making it harder for them to reject sperm. Each dart-using species has its own particular shape and style of dart, several of which can be seen below.

One might argue that a hormone-manipulating injection isn't exactly a gift, but Lewis chalks that up to "the complexities of the English language. If you look up definitions of a gift, it's voluntarily given, but it doesn't need to be voluntarily received. We've all received gifts we didn't want."

Bowerbird Love-Nests

Drab in plumage, male bowerbirds rely on another type of ornamentation to woo mates: They build complex pavilions, something like avian honeymoon suites but with bones, pebbles and shells instead of rose petals strewn across the floor.

Females, which breed only once per year, tour their neighborhoods, inspecting the males' creations and choosing the most talented architects for mating. Sex occurs inside the pavilion, with females then leaving to build their own nests.

What exactly impresses the lady bowerbirds? Scientists aren't sure, but some think that the males' creations, technically known as gessos, are literally hypnotizing.

A Spider's Sweet Nothings

Among the best-studied of gift-giving animals are Paratrechalea ornata, a spider species found along rivers and streams in much of South America. Male P. ornatapresent females with silk-wrapped prey parcels; the larger the gift, the better their chances of mating, and the longer they spend copulating. For the male, this is doubly beneficial: Time she spends mating with him is time that isn't spent mating with his competitors.

It's even been suggested that the gifts might remind females of egg sacs, exploiting her maternal instincts — softening her up, so to speak, by making her think about babies.

It's indeed better not to die before mating. To this end, male nursery-web spiders, driven by the reproductive imperative but wary of cannibalistic females, present them with silk-wrapped food bundles, then fall over and pretend to be dead. Once she's started to eat, the male comes to life and initiates copulation.

A Firefly's Pasta Dinner

Males of many species produce what are called spermatophores, or gelatinous packets that contain both sperm and nutrients for their mates. Those made by fireflies are especially elegant.

"Nobody had any idea that fireflies were giving a nuptial gift. They're just really beautiful," said Lewis of her discovery. "They look like rotini."

Spermatophores are especially important for fireflies, which don't feed as adults. For females, they're a lone source of nutrition, and become more important as a breeding season progresses. This eventually flips the script on mating dynamics, creating one of the few examples where males rather than females are in high demand.

"Females are forced to rely on the male gifts for feeding eggs at the end of their lives," said Lewis. "They get to be very desperately seeking males. At the end of a season, you'll see a male fly across a field, and whereas females are normally unresponsive, the whole field will light up. They're down there in the grass, looking for males."

Thoughtful Jays

Among Eurasian jays, a common member of the bird family that includes crows, ravens and blue jays, males give gifts of food to their mates. Like many humans, they're no longer trying to impress potential lovers, but strengthening the bonds between lifetime partners.

Salamander Gift Delivery

It's not only insects that produce spermatophores. So do many salamanders. Sometimes females eat them while mating (above); at other times they're left at the bottom of vernal pools, where they're an easy-to-spot sign of salamander activity, for females to pick up later.

During mating, male six-spots make a gift of cyanide, transferring it to females for use in their own defense. Females also transfer cyanide into their eggs, bequeathing larvae with an helpfully poisonous inheritance.

Cheap Scorpionflies

To attract partners, male scorpionflies produce pheromone- and nutrient-laden spitballs on which females feed as they mate. Some males, however, run out of spit. "They actually offer females a dead insect instead," said Lewis. "A lot of times they'll re-use the same insect, again and again. By the end, it's been sucked dry."

These males enjoy less reproductive success than their more prodigious brethren, Lewis said.

Squid Spears

In many species of squid, sperm is transferred in complex spermatophores that function like harpoons. Exactly how the spermatophores are delivered is a matter of debate: Some biologists think they're contained inside the penis, but others aren't so sure. Squid are notoriously difficult to study in their deep marine habitats.

At least for hooked squid, some of the mystery is known. Males are literally covered in the spermatophores (above) which lodge in female skin, slowly working their way in (below) before releasing their payload.

Not-So-Romantic Chimpanzees

Given the ubiquity of nuptial gift-giving among animals, one would expect to find examples in mammals. Instead it proves to be rare, with humans unique in giving gifts. Even in chimpanzees, our closest living relative, nuptial gift-giving doesn't seem to exist, though there does seem to be a long-term relationship between food provisioning and sex.

According to Lewis, the absence of mammalian nuptial gift-giving doesn't necessarily mean it's not there. The phenomenon has received relatively little attention from researchers. "It's definitely possible that we just haven't looked hard enough," she said.

It's Good to Be Generous

Spermatophores are made for female consumption by males of many cricket species, but the quality of these gifts varies widely.

Some contain sperm-protecting compounds and few nutrients. They're easy to make, allowing males to breed several times per night over the course of their three-week lives, but offering little benefit to females

Other spermatophores are highly nutritious, but they're also difficult to produce. Males who do mate just once or twice before dying. Why opt for such a resource-intensive strategy? These spermatophores also contain compounds that encourage females to lay more eggs and discourage her from mating with other males.